U.S. Pat. No. 5,081,639 teaches a laser diode assembly including a cylindrical lens. The laser diode assembly taught therein includes a diffraction-limited cylindrical lens having a numerical aperture greater than 0.5 which is used to collimate a beam from a laser diode. A collimated beam is one which is neither converging nor diverging; i.e., the rays within the beam are travelling substantially parallel to one another. Laser diodes are efficient sources of laser radiation, however the highly divergent beam emitted from a laser diode presents problems in many applications. The divergence of the laser diode's beam is caused by its exit aperture which is very narrow along one axis (the "fast" axis), and much wider along the other (perpendicular) axis. These two axes correspond to the X and Y axes, as will be later explained. The cross section of the beam emitted along the fast, or Y, axis is highly divergent due to diffraction effects. In comparison, the wider aperture, defined along the X axis, emits a beam cross section that diverges only slightly.
In order to collimate the beam produced by a laser diode, the invention taught in U.S. Pat. No. 5,081,639 teaches the mounting of a cylindrical lens optically aligned with the laser diode to provide a beam of collimated light from Y axis of the diode.
U.S. Pat. No. 5,181,224 illustrates the use of cylindrical lenses to (inter alia) create a slowly diverging beam light. This lens may be said to be "circularizing" and, when installed on any of a variety of laser diodes is available as the "CircuLaser" diode available from Blue Sky Research in Santa Cruz, Calif.
While the above-described laser diode assemblies are fully effective for their intended use, the method of manufacture has heretofore resulted in manufacturing inefficiencies. In any optical system, the alignment of the various optical elements is critical to the functioning of the system. This is certainly the case where a cylindrical microlens is incorporated into an optical system with a laser diode to provide a low-cost source of collimated light. As is typical of many optical applications, there are six degrees of freedom inherent in the positioning of the lens with respect to the laser diode, as shown in FIG. 1. Having reference to that figure, an optical element in the form of a cylindrical microlens 100 is schematically shown. The lens has three axes, X, Y and Z. The Z axis, 1, corresponds to the optical axis of the optical system. The X, 3, axis is transverse to the Z axis, 1, in the horizontal plane. The Y, 2, axis is also perpendicular to the Z axis but in the vertical direction. Positioning the lens along the X, Y, and Z axes defines the first three degrees of freedom. Furthermore, the lens may be rotated about each of these axes as shown at 10, 20, and 30, and each of these rotations also defines a degree of freedom with regard to alignment of the lens in the optical system. For cylindrical lenses, placement of the lens along the X axis, 3, is not critical. This fact means that the alignment of a cylindrical microlens with respect to a laser diode accurately requires alignment with five degrees of freedom.
It will be apparent to those of ordinary skill in the art that a mechanical translation stage providing the required five degrees of freedom is subject to considerable inaccuracies. These inaccuracies are the cumulative result of the tolerances required by any mechanical system for motion in essentially five directions.
To overcome this source of error, the manufacture of laser diode assemblies including microlenses has, to date, proceeded in a number of steps. First, a section of cylindrical microlens is mounted on a small mounting bracket which because of its resemblance to a football goal post is referred to as a "goal post." It is intended that rotation about the X and Y axes is defined by the lens' position on the goal post. After the lens is mounted on the goal post, the goal post/lens assembly is then optically positioned along the Z axis, and the lens affixed to the laser diode. In this manner, movement along the several axes, as well as rotation about those axes is manipulated by an operator who assembles each lens and laser diode. The entire operation is very dependent on the skill of the operator, as the optical cement utilized first to affix the lens to the goal post and finally to the diode introduces a variable into the problem. This variable is simply that the surface tension of the cement between the several elements on which it is used causes motion between those elements. This motion of course tends to misalign the optical elements.
From the foregoing discussion, it is apparent that the current manufacturing process for laser diode assemblies including a cylindrical microlens is a labor-intensive process, requiring considerable effort on the part of skilled technicians to effect the assembly of one lens to one diode.
What is needed is a methodology which will result in substantial savings in skilled manpower required to accurately assemble a cylindrical microlens or other optical element with a laser diode.
Laser diodes, or more properly, semiconductor lasers, are generally constructed according to well known principles of semiconductor manufacturing technology. A discussion of these principles can be found in Richard R. Shurtz II, Semiconductor Lasers and LEDs in Electronics Engineers' Handbook, 3rd ed. (Donald G. Fink and Donald Christiansen, eds. 1989). Semiconductor laser manufacturing technology comprises a number of broad steps as discussed in the cited reference. These manufacturing steps may be broadly summarized as crystal growth; wafer cutting and polishing; epitaxial growth; masking; passivation; diffusion; metalization, thick and/or thin film deposition; patterning and etching; the wafer sort process; cleaving and/or sawing the laser wafer into individual dice; and finally, assembly and testing of the dice. The process of cleaving and/or sawing the laser diode wafer into individual dice holds out the possibility for a partial solution to the previously discussed problem.